1. Technical Field
This invention relates to polymers. More particularly, it relates to processes for the preparation of Ziegler-Natta catalysts useful for preparing polymers.
2. Background of the Art
Olefins, also called alkenes, are unsaturated hydrocarbons whose molecules contain one or more pairs of carbon atoms linked together by a double bond. When subjected to a polymerization process, olefins may be converted to polyolefins, such as polyethylene and polypropylene. One commonly used polymerization process involves contacting an olefin monomer with a Ziegler-Natta type catalyst system. Many Ziegler-Natta type polyolefin catalysts, their general methods of making, and subsequent use, are well known in the polymerization art. Typically, these systems may include a Ziegler-Natta type polymerization catalyst component; a cocatalyst; and an electron donor compound. A Ziegler-Natta type polymerization catalyst component may be a complex derived from a halide of a transition metal, for example, titanium, chromium or vanadium, with a metal hydride and/or a metal alkyl that is typically an organo-aluminum compound. The catalyst component is usually comprised of a titanium halide supported on a magnesium compound complexed with an alkylaluminum. There are many issued patents relating to catalysts and catalyst systems designed primarily for the polymerization of propylene and ethylene that are known to those skilled in the art. Examples of such catalyst systems are provided in, for example, U.S. Pat. Nos. 4,107,413; 4,294,721; 4,439,540; 4,114,319; 4,220,554; 4,460,701; 4,562,173; 5,066,738; and 6,174,971, which are fully incorporated herein by reference.
Conventional Ziegler-Natta catalysts comprise a transition metal compound generally represented by the formula MRx, where M is a transition metal compound, R is a halogen or a hydrocarboxyl, and x is the valence of the transition metal. Typically, M is selected from the Group IV to VII metals, such as titanium, chromium or vanadium, and R is chlorine, bromine, or an alkoxy group. Common transition metal compounds may be TiCl4, TiBr4, Ti(OC2H5)3Cl, Ti(OC3H7)2Cl2, Ti(OC6H13)2Cl2, Ti(OC2H5)2Br2, and Ti(OC12H25)Cl3.
Ziegler-Natta catalysts generally may be provided on a support, e.g., deposited on a solid crystalline support. The support may be an inert solid, which is chemically unreactive with any of the components of the conventional Ziegler-Natta catalyst. The support is often a magnesium compound. Examples of the magnesium compounds which may be used to provide a support source for the catalyst component may be magnesium halides, dialkoxymagnesium, alkoxymagnesium halides, magnesium oxyhalides, dialkylmagnesiums, magnesium oxide, magnesium hydroxide, and carboxylates of magnesium.
The properties of the polymerization catalyst may affect the properties of the polymer formed using the catalyst. For example, polymer morphology typically depends upon catalyst morphology. Good polymer morphology includes uniformity of particle size and shape and an acceptable bulk density. Furthermore, it is desirable to minimize the number of very small polymer particles (i.e., fines) for various reasons, such as for example, to avoid plugging transfer or recycle lines. Very large particles also must be minimized to avoid formation of lumps and strings in the polymerization reactor.
Thus, it would be desirable in the art to improve and/or to control catalyst morphology such as to, by extension, improve polymer morphology.